Applying Six Sigma Process Capability to Safety

The first step is to establish the upper and lower specification for the air handling system. The Upper Specification Level is the upper acceptable tolerance level for the air handling system. In this example, the desired average level is 20 ppm while the system is designed to maintain that level plus or minus. 01 ppm. The Upper Specification Level is determined using the desired average level of 20 ppm +. 01 ppm = 20.01 ppm while the Lower Specification Level would be 20 ppm -. 01 ppm = 19.99 ppm. 

The obtained Z value of 1.5 is unacceptable since a minimum value of 3.0 is desired. A Zy value of 1.5 indicates, using a Z table, that approximately 7% of the readings are expected to be above the upper specification level of 20.01 ppm. 

A Zl value of -. 5 indicates, using a Z table, that approximately 30% of the readings would be below the lower specification level, which in this example, would indicate that exposure to the air contaminants would be even less than expected. 

The next step is to determine the Capability Index (Cpk) for the system. Cpk is calculated by using the smaller value of either the Upper Process Location (Z ) or Lower Process Location (ZJ. In this example, Z is the smaller value with -. 5, therefore, the Process Capability (Cpk) is calculated and interpreted as follows  

Since the result of -. 16 is less than 1.00, which is required, and 1.33, which is preferred, the Capability Index indicates that the air handling equipment is not capable of meeting the requirements of controlling the air contaminant levels to a sufficient level as defined by the process specifications. In approximately 37% of the samples would be expected to be outside of the desired upper and lower specification ranges. While the lower readings in this example would be beneficial, the readings above the upper level could pose increased risks. 

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